In recent years, hybrid vehicles have drawn public attention due to a demand for low fuel consumption and low emission vehicles. Generally, hybrid vehicles utilize an internal combustion engine and a motor generator as power sources. In some hybrid vehicles, as disclosed in a patent document 1 (i.e., a Japanese Patent No. 3812134), the engine power is determined based on a drive power (i.e., a power requested for a travel of the vehicle) and a battery charge-discharge electric power so that a battery charge state is maintained within a certain preset range.
In a hybrid vehicle, a fuel consumption per mile is reduced by an EV drive that solely drives the hybrid vehicle via power from an electric motor without using the internal combustion engine, that is, by stopping the engine due to a low efficiency operation state of the engine at a low driving load. However, using the EV drive for too long lowers a battery State Of Charge (SOC), and the EV drive may have to be interrupted for the engine drive of the vehicle, which does not improve fuel mileage. Therefore, when the battery SOC lowers to a certain level, the battery is charged by using the engine power, that is, by generating electricity for the charging of the battery by the operation of the engine.
However, if the amount of electricity generation during a high driving load is set to a high level for a quick recovery of the battery SOC, for example, the high level setting of the electricity generation amount may lead to a deterioration of system efficiency. That is, an efficiency of a system as a whole, i.e., a hybrid vehicle as a system which includes the engine, the generator, the battery and the like, may be deteriorated by such setting at the high driving load, thereby preventing a sufficient improvement of the system efficiency and preventing the improvement the fuel mileage.
Further, in the hybrid vehicle, the amount of heat may be insufficient in winter, i.e., in a heating operation time in winter, due to a low-frequency operation of the engine. Such an insufficient heat amount situation may be quickly resolved by increasing the amount of the engine-generated electric power from the engine, which eventually increases the amount of heat from the engine. However, the amount of electric power generation may have to be carefully determined in consideration of the system efficiency, for the improvement of the fuel mileage.
Therefore, in a hybrid vehicle, in order to effectively improve the fuel mileage, the efficiency of the system, including the engine, the generator, the battery and the like, as a whole must first be improved. However, the technique in the above-described patent document 1 does not consider the system efficiency. That is, the technique in the patent document 1 simply determines an engine power for keeping the battery SOC in a certain range. Thus, it may be difficult for such technique to improve the system efficiency and to effectively improve the fuel mileage.